The invention is generally directed to a circuit substrate and the method for manufacturing the circuit substrate and in particular to a circuit substrate including bumps on the finger leads with specified surface roughness for connection with the conducting leads of a semiconductor element (hereinafter referred to as an "IC chip") which is designed to be mounted on the circuit substrate. The surface roughness of the bumps can be augmented by plating the surface of the bump. The circuit substrate is used to manufacture integrated circuit devices by the strip-support method which utilizes a roll of circuit substrate which acts as a carrier for many IC chips which are attached to the roll of substrate one after the other at high speed. The invention is also directed to the method of forming the bumps on the circuit substrate to aid in mechanically and electrically connecting the substrate to the electrodes of the IC chip or other electrical device.
Reference is made to FIG. 1 wherein a cross sectional view of a prior art circuit substrate including bumps for manufacturing integrated circuits in accordance with the strip-support method is depicted. The circuit substrate includes a circuit substrate base film 1 and a conductive pattern 3 coupled to base film 1 with an adhesive 2. Conductive pattern 3 has a finger lead portion 4 extending inwardly from base film 1 to make contact with an IC chip (not shown). Finger lead 4 has a narrowed or concave neck portion 5 and a bump portion 6. Contact surface 8 of bump 6 is adapted to be connected with the aluminum pads of an IC chip and is generally flat so that the roughness (distance between the highest point and the lowest point on the surface) is less than 3 microns. Japanese Patent Publication Nos. 58/26828 and 59/17980 generally disclose substrates with bumps formed at the end portions of the finger leads by plating or by half etching the middle portion of the finger lead. Although these references do not specifically refer to the characteristics of contact surface 8, the surfaces are generally smooth.
The prior art method of forming finger lead 4 exposes the front and back surfaces of the conductive layer to form conductive pattern 3 separately. As a result, the positioning of the substrate for exposure after several circuit substrates on the roll have been exposed is likely to be inaccurate which makes it difficult to form bump 6 accurately at a predetermined position on finger lead 4. The prior art method is shown in FIGS. 2A-2G described below in greater detail.
The prior art substrate structure suffers from several deficiencies. When the IC chips are attached by attempting to bond the aluminum pads on the IC chips to the substrate bumps, the connection between the IC chip and the substrate is often unstable, weak or absent, particularly when the oxide film on the surface of the aluminum pad is relatively thick. Generally, the aluminum pad of the IC chip and the bump of the finger lead are connected by thermal contact bonding, sometimes with a supersonic wave. However, the smooth surface of the bumps contacting the aluminum pad cannot destroy and remove the oxide film formed on the surface of the aluminum pads, thereby causing a weakened and unreliable contact between the substrate and the IC chip.
The prior art method of forming the bumps on the substrate disclosed in Japanese Patent application Ser. No. 59-17981 is shown in FIGS. 2A-2G. Like elements in the Figures are represented by like reference numerals. As shown in FIG. 2A, the front surface of conductive layer 20 is coated with a protective resist 71. Next, as shown in FIG. 2B, the back surface of conductive layer 20 is coated with a photo resist 30 and bump 6 is formed by exposure, development and half-etching in accordance with standard practice. Next, as shown in FIG. 2C, protective resist 71 and photo resist 30 on the front and back surfaces of conductive layer 20, respectively, are removed. Then, as shown in FIG. 2D, the front surface of conductive layer 20 is coated with a photo-resist 31 and patterned by exposure and development. Next, as shown in FIG. 2E, the back surface of conductive layer 20 is coated with a protective resist 72. As next shown in FIG. 2F, the front surface of conductive layer 20 is etched to form the circuit pattern including finger lead 4. Finally, as shown in FIG. 2G, photo-resist 31 and protective resist 72 are removed from the front and back surfaces of conductive layer 20, thereby forming bump 6 on the back surface of finger lead 4 around device opening 40.
However, this method has several defects. Since the front and back surfaces of conductive layer 20 are separately exposed, the positioning of the substrate for a series of exposures is likely to be inaccurate, which makes it difficult to form bump 6 accurately at a predetermined position on finger lead 4. Especially where the substrate is automatically exposed in a successive manner such as is the case in forming a substrate for use in the strip-support integrated circuit production method, the patterns on the front and back surfaces of the substrate can only be aligned in registration with each other initially. It is then impossible to assure proper alignment of the patterns at later positions along the strip. Thus, if the accuracy of positioning the substrate by the exposure apparatus is not exact or if the sprocket holes 50, which guide the movement of the substrate, change or are poorly placed, many defects, such as the bump being deformed and the bump being positioned incorrectly on the substrate can result. The positioning error between the bump on the substrate and the IC chip electrode also results in mechanical and electrical connection problems.
In addition, the prior art method half-etches conductive layer 20 which somewhat reduces its strength. Thereafter this weakened layer is then coated with photo-resist, exposed, developed and subjected to other processes which make it likely that the conductive layer will be negatively affected. As a result, cracks in the photo-resist can cause wiring discontinuities which makes such substrates useless. These problems in a production environment may not be discovered until the IC chip has been attached, increasing the cost of the error.
Accordingly, there is a need to provide an improved method for forming the bumps on the conductive layers of the circuit substrates where the positioning error of the bumps on the finger leads are prevented and the process is simplified to reduce damage to the conductive layers so that reliable formation of a desirable patterns is provided. There is also a need to provide an improved circuit substrate with bumps that have a surface roughness which destroys the oxide layer on the IC chip conductive pads to form a stronger and more reliable connection. There is also a need to plate the bumps to increase the surface roughness of the bumps.